The long term goal of the proposed experiments is to determine the causes of attention impairment during disease. However, it is first necessary to understand the organization of circuits that may contribute to attention mechanisms in the normal brain. Many studies have implicated the pulvinar nucleus as an area that may regulate visual attention. We propose to study the rodent lateral posterior (LP) nucleus, a region comparable to inferior regions of the primate pulvinar nucleus. We will use a combination of tract tracing, immunocytochemistry, electron microscopy and in vitro electrophysiology to examine the synaptic organization and functional properties of LP neurons. We propose that the rat LP nucleus contains at least 2 subdivisions that are characterized by distinct synaptic inputs, axonal projection patterns, and cellular firing properties. Regular spiking cells in the caudal LP nucleus are predicted to receive tectal input and project to the striatum. The synaptic arrangements of tectothalamic terminals, coupled with the linear firing properties of caudal LP neurons, may serve to accurately transfer signals related to the movement of visual stimuli. This circuitry may underlie the coordination of visually guided movements. Clustered spiking cells in the rostral LP nucleus are predicted to receive input from layer 5 of the visual cortex and in turn project to cortical layer 1. The synaptic arrangements of layer 5 corticothalamic terminals, coupled with the non-linear firing properties of rostral LP neurons, may serve to amplify and synchronize cortical activity, providing a substrate for the selective enhancement of visual signals. A thorough understanding of the firing characteristics of LP neurons and the organization and dynamic properties of their synaptic connections is necessary to begin to formulate reasonable hypotheses regarding the function of the rat LP nucleus, and ultimately the human pulvinar nucleus. [unreadable] [unreadable]